A number of electronic ignition systems have been developed to provide spark ignition for internal combustion engines. Among them, capacitor discharge systems, in which a capacitor is charged to a relatively high voltage and then rapidly discharged by a thyristor such as a silicon controlled rectifier (SCR) through a step-up ignition transformer, have been highly satisfactory.
One such system is disclosed in U.S. Pat. No. 4,015,564 to the present inventor, entitled "Ignition System For Internal Combustion Engines Having Timing Stabilizing Means". In that system the main capacitor has one side connected to engine ground and the other side connected to the anode of the controlled rectifier. The cathode of the controlled rectifier is connected to the ignition transformer, the other side of which is grounded. A triggering circuit utlizes timed pulses generated in a trigger coil by a magnet coupled to the engine flywheel to trigger a pilot silicon controlled rectifier which in turn is transformer coupled to the gate of the main controlled rectifier to discharge the main capacitor. Such an arrangement is particularly useful where an electrically positive discharge pulse is desired since it allows the ignition transformer to use a common ground between its primary and secondary coils, and further, allows the triggering signals to be relative to ground. That system, however, required a trigger pulse transformer to couple the pilot SCR to the gate of the main SCR since the cathode of the main SCR discharges to the ignition coil.
A capacitor discharge ignition system disclosed in U.S. Pat. No. 3,739,759 to Sleder and entitled "Rotation Sensing Pulse Control Generator For Triggered Ignition Systems" shows a triggering system having a pilot SCR directly coupled to the gate of the main ignition SCR. In this system, however, the cathode of the main SCR is directly connected to ground and a negative output pulse is provided to the ignition transformer. Such a system would not be suitable where a positive output pulse is required, as for example, for use in the system described in the present inventor's copending U.S. Patent application Ser. No. 330,419, entitled "Capacitor Discharge Ignition System Having a Charging Control Means", filed on the same date as this application, now Pat. No. 4,433,668.
Another ignition system having a pilot SCR directly coupled to a main SCR is shown in U.S. Pat. No. 3,937,200 to Sleder and the present inventor entitled "Breakerless and Distributorless Multiple Cylinder Ignition System". That system uses two discharge circuits controlled by a single SCR. In one of the discharge circuits the anode of the SCR is connected through a diode to ground while the cathode is connected to an ignition transformer which in turn is connected through an energy-storage capacitor to ground. In this discharge circuit the cathode of the SCR will be charged negatively and rise to ground as the capacitor discharges. In the other discharge circuit the cathode of the SCR is connected through a diode to ground. Consequently, the cathode of the SCR cannot rise above ground to inhibit the gate signal. This arrangement, however, maintains a negative potential for substantial periods of time on the trigger coil. Thus, any inadvertent leakage in the trigger coil circuit could cause untimely triggering of the discharge circuit. Further, the system does not permit the ignition transformers to use a common grounded connection between the primary and secondary coils.